Current day use of monocomponent nonmagnetic development systems in electrophotographic devices is exemplified by the following publication items:
[1] H. Sato et al., Oki Electric Industry Co., Ltd., "Contact Development With Nonmagnetic Monocomponent Toner," The Sixth International Congress on Advances in Non-Impact Printing Technologies, The Society for Imaging Science and Technology, Springfield, Va., 1990, pp. 76-77 (Advance Printing of Paper Summaries);
[2] A. Shinozaki et al., Ricoh Co., Ltd., "Influence Of Electric Characteristics Of Development Roller Used In Non-magnetic Single-Component and Contact Development Process," ibid., p. 10 (Advance Printing of Paper Summaries), and pp. 55-61 (full text);
[3] J. A. Thompson, IBM Corp., "A Review Of The Development Process Technology Utilized In The IBM LaserPrinter Family," ibid., pp. 11-12 (Advance Printing of Paper Summaries), and pp. 72-84 (full text);
[4] M. Lee et al., IBM Corp., "Charge Distribution Of Toner In Jump Development," ibid, p. 75 (Advance Printing of Paper Summaries), and pp. 196-206 (full text);
[5] H. Yamamoto et al, Matsushita Electric Industrial Co., Ltd., "Novel Color Electrophotography: `One Drum Color Superimposing Process`," The Fifth International Congress on Advances in Non-Impact Printing Technologies--Proceedings, The Society for Imaging Science and Technology, Springfield, Va., 1990, pp. 115-128; and
[6] Matsushita Electric Industrial Co., Ltd., Panasonic FP-C1 Service Manual, Section IV (undated), pp. 4-17 and 4-07.
In a typical electrophotographic device using monocomponent nonmagnetic developer (toner) to develop an electrostatographic (electrostatic) latent image on a photosensitive surface of a photoconductor (drum), a toner adder roller (foam roller or brush roller) applies the toner to a developer roller (sleeve) and a doctor blade smooths it into a thin layer for transfer to the photosensitive surface to develop the latent image. Friction contact with various surfaces including the adder roller, developer roller and doctor blade is required to charge the toner triboelectrically to develop the latent image.
This technology is primarily used in black-only devices such as laser printers However, there is great interest in using monocomponent nonmagnetic development for low-cost color printers.
One typical development system, as disclosed in item [1] above, uses a conductive elastomeric developer roller having a non-conductive outer coating. Toner is applied onto this developer roller by a foam type toner-adder roller. A regulation (doctor) blade in contact with the developer roller smooths the resulting layer of applied toner. The image is developed with the developer roller in contact with the photoconductor.
Similar development systems are used for non-contact development, as disclosed in items [4], [5] and [6] above. These systems have a conductive, nonmagnetic, metallic developer roller spaced a few mils from the photoconductor.
Item [2] above concerns a so-called particle electrode developer roller (sleeve) having a conductive rubber substrate coated with a carbon or other particle containing insulating layer (electrically isolated electrode particles or carrier particles in a non-conductive resin matrix, i.e., floating electrodes).
In the IBM Model 4019 laser printer (IBM 4019), which is similar to that disclosed in item [1] above, the toner is charged negatively by triboelectric charging against the non-conductive coating of the developer roller. It is also charged by negative DC bias voltages on the conductive foam toner adder roller and metal doctor blade used in this device. The toner is held to the developer roller by attraction to triboelectric charges on the developer roller surface, image charges, surface charges and toner adhesion, until development of the latent image occurs (item [3] above).
In particular, item [3] above reviews the operation of an IBM laser printer (assumably the IBM 4019) with contact developing of nonmagnetic thermoplastic (insulative) toner only on the discharged latent image area of a drum type photoconductor. The developer unit includes an adder roller and doctor blade each in friction contact with an elastic semiconductive developer roller.
The adder roller has an open cell urethane foam substrate of 40 pores/inch, overcoated with a conductive layer to yield a low bulk resistivity of about 10.sup.4 ohm cm or less, on which toner deposits. The adder roller runs at about 2.5 times the print speed for charging the toner against the developer roller and creating a counter-charge on the developer roller causing toner to adhere thereto. A DC bias is applied to the adder roller which is 100 VDC more negative than the DC voltage applied to the developer roller, to aid toner loading and friction charging on the developer roller as well as discharging of residual charge after image development.
The doctor blade has a surface treated with tungsten carbide particles and is held under an 800 gram (34.5 g/cm) force to form a monolayer of toner on the developer roller and add charges of the desired polarity to the toner layer. A DC bias is applied to the doctor blade which is 325 VDC more negative than the DC voltage applied to the developer roller.
The developer roller has a 6 mm thick nitrile rubber, elastic layer of about 10.sup.9 ohm cm resistivity and 45 Shore A hardness, overcoated with a 50 micron thick polyurethane (insulating) outer coating which is triboelectrically active with the toner. The elastic semiconductive developer roller acts like the carrier particles in a two-component developer of toner and carrier.
All monocomponent nonmagnetic development systems offer the advantage of no toner concentration monitor, no carrier pick-up onto the photosensitive surface (film) of the photoconductor, and no carrier aging or replenishment concerns, as are common in two component development systems, i.e., using a particle mixture of toner and carrier. Monocomponent nonmagnetic development technique is attractive for low cost color printers as there is no magnetic material in the toner to interfere with color applications. However, for triboelectrically charging the toner, the prior art relies on contact with the doctor blade and the developer roller surface, and to some extent on contact with the toner adder roller.
Triboelectric charging by contact with adjacent surfaces makes the process sensitive to surface contamination, e.g., toner scumming, which adversely affects performance. Scumming is the permanent adhesion of fine toner particles to the developer roller and doctor blade due to frictional contact during triboelectric charging of the toner and leads to image quality defects.
Triboelectric charging of toner by contact with adjacent surfaces also limits the system to either a thin layer of toner on a developer roller running at modest speed, or poor control of charging which results in a broad toner charge to mass ratio (Q/M) distribution, deposition of toner in image background areas, and cleaning problems. Moreover, in the case of the IBM 4019 laser printer, the non-conductive developer roller surface, which is used for triboelectrically charging the toner, itself becomes charged and must be discharged after development occurs.
For development with the developer roller in contact with the photoconductor, as disclosed in item [1] above, background toner deposition can be reduced if the roller surface (peripheral) speed is faster than the photoconductor surface speed In the IBM 4019 laser printer, which uses contact development, the developer roller surface speed is about 1.5 times that of the photoconductor.
Additional problems occur if this development process is used to print gray scales, high density solid areas on light density backgrounds, or black on gray images, as required for a color printer. Particular problems involve:
(a) White halo surrounding black solids on gray backgrounds. PA1 (b) Dark fringe or edge development of gray solids on white backgrounds, caused by image fringe fields with contributions from changes in developer roller polarization at solid area edges. PA1 (c) Double printing of black images in gray backgrounds, as a result of differential speed contact development, e.g., a 1.5:1 developer roller to photoconductor surface speed ratio. This causes high charge to mass (Q/M) toner on the developer roller to develop first, and low charge to mass remainder toner to develop background gray areas after the developer roller advances from a black area to an undeveloped gray region of the latent image. PA1 (d) High contrast reflection density (Dr) versus development potential characteristics, as caused by cooperative development due to toner cohesion. If one toner particle develops, others follow (item [4] above).
Development potential (delta V) is the voltage difference between the DC voltage of the photoconductor latent image and that applied to the developer roller, i.e., the photoconductor latent image voltage minus the developer roller voltage.
Because of these problems, the prior art cannot provide an effective gray level, or continuous tone color, printer, e.g. a laser printer, for monocomponent nonmagnetic developer systems.
It is noted that in discharge area development (DAD), as used in laser printers, the charge on the latent image area is discharged while the image background area remains charged, and the toner is charged to the same polarity as the background area, being repelled therefrom and attracted to the discharged latent image. Conversely, in charge area development (CAD), as used in electrophotographic copying machines, the charge on the background area is discharged while the latent image remains charged, and the toner is charged to the opposite polarity to that of the latent image, being preferentially attracted to the latent image.
Examples of systems for developing an electrostatic latent image are shown in the following prior art.
U.S. Pat. No. 4,450,220 (Haneda et al.) and its division U.S. Pat. No. 4,675,267 concern cloud charging of nonmagnetic or magnetic toner, or two component developer, in an AC field between a developer roller as one electrode and a fixed plate as the other electrode, each under an AC, and optionally a DC, bias. Toner is charged at least in part by alternately impinging against the electrodes, and deposits on the developer roller for non-contact developing. The nonmagnetic toner may be blended with silica powder, and the carrier of the two component developer may be insulating material such as glass beads.
U.S. Pat. No. 5,034,775 (Folkins) concerns applying two component magnetic developer to a magnetic developer roller under a DC bias for powder cloud developing. Toner is attracted from the developer roller to a spaced donor roller under a higher DC bias, for transfer to the photoconductor via a gap having powder cloud generating electrodes under a low AC bias. The electrode AC bias provides an alternating field with the donor roller to form the cloud, and the donor roller DC bias provides an electrostatic field with the photoconductor to attract toner thereto.
The following prior art concerns developer units that do not include an adder roller.
U.S. Pat. No. 5,041,351 (Kitamori et al.) concerns use as a monocomponent magnetic developer of a mixture of negatively chargeable magnetic toner particles, positively chargeable fine resin particles such as polymethylmethacrylate (PMMA), and negatively chargeable silica powder. The mixture is applied to a magnetic developer sleeve for non-contact developing under an AC bias applied between the sleeve and photoconductor.
U.S. Pat. No. 4,653,426 (Kohyama et al.) discloses applying magnetic toner or two component developer to a developer roller for non-contact developing of various size and charge level toner particles, under a developer roller DC bias and cyclically varying multiple frequency AC bias.
U.S. Pat. No. 4,528,936 (Shimazaki et al.) and U.S. Pat. No. 4,586,460 (Kahyama et al.) disclose applying nonmagnetic toner, optionally with a flow improver, to a developer roller for non-contact developing, optionally under a developer roller DC or AC bias.
U.S. Pat. No. 4,395,110 (Hosono et al.) discloses applying nonmagnetic or magnetic toner, optionally mixed with silica particles, to a developer roller for non. contact developing under a DC and AC bias applied between the roller and photoconductor.
U.S. Pat. No. 5,043,239 (Kokimoto et al.) concerns use as a monocomponent magnetic developer of a mixture of negatively chargeable magnetic toner particles and treated silica powder. The mixture is applied to a developer sleeve containing a magnet, and used for reversal developing under an AC and DC bias applied between the sleeve and photoconductor.
U.S. Pat. No. 4,100,884 (Mochizuki et al.) discloses applying nonmagnetic or magnetic toner to a developer roller and forming it into a layer by a doctor blade, per friction contact charging or use of a scorotron charger. Non-contact developing is effected under a bias voltage applied via a switch to the developer roller. The roller has a coating of one type for positively charging the toner and another for negatively charging the toner.
The following prior art concerns developer units that do include an adder roller.
European Application No. 241,160 A2 (Shinya et al.) discloses applying nonmagnetic toner by an unidentified member, assumably an adder roller, to a developer roller for non-contact developing under a developer roller DC and/or AC bias. The toner is a positively chargeable resin, surface treated with a silane agent.
U.S. Pat. No. 4,903,634 (Ono et al.) and its counterpart European Patent No. 205,178 B1 disclose applying nonmagnetic or magnetic toner by an adder roller, against which the toner supply is loaded, to a developer roller for non-contact developing under a developer roller DC bias. The adder roller is under a DC, or a DC and AC, bias, and contacts the developer roller, an excess toner removing plate, and the unit wall, for charging the toner.
U.S. Pat. No. 5,012,285 (Oku et al.) discloses applying monocomponent or two component magnetic developer by an adder roller to a developer sleeve containing a magnet, and forming it into a layer by an agitating doctor blade. Non-contact developing occurs under a sleeve DC bias, with the adder roller being spaced from the sleeve and under a DC and AC bias.
U.S. Pat. No. 4,286,543 (Ohnuma et al.) discloses applying nonmagnetic or magnetic toner or two component developer, to a developer roller and forming it into a layer by a doctor blade. The developer roller is under a DC bias of opposite polarity to the toner, and the doctor blade is under a different DC bias of the same polarity as the toner. Optionally, the toner or developer is applied to an adder roller, formed into a layer by the doctor blade, and then transferred to the developer roller. The developer roller has a conductive coating optionally overcoated with an insulating outer coating, and has a magnet for magnetic brush development when using a magnetic toner or developer. Contact development is effected under the developer roller DC bias.
British Application No. 2,197,227 A (Hirano et al.) discloses applying nonmagnetic or magnetic toner associated with silica or other "metal" oxide, by an adder roller to a developer roller. It is formed into a layer by a silica filled silicone rubber doctor blade for contact developing. The metal oxide of the toner is adsorbed on the doctor blade to inhibit toner fusion thereto.
U.S. Pat. No. 4,760,422 (Seimiya et al.) discloses applying nonmagnetic toner, optionally containing a flow improving inorganic powder, to a particle electrode developer sleeve. For contact developing, the sleeve is under a DC bias, and for non-contact developing it is under a pulse voltage, or an AC, or AC and DC, bias, the AC having a smaller amplitude than the gap distance between the sleeve and photoconductor.
U.S. Pat. No. 4,696,255 (Yano et al.) and analogous British Patent No. 2,163,371 B (Demizu et al.) disclose applying nonmagnetic toner by an adder roller to a particle electrode developer sleeve. It is formed into a layer by a doctor blade and effects developing under a sleeve voltage bias. The sleeve has a particle electrode-containing insulating layer of a resin spaced in the triboelectric series from the toner for charging the toner relative to the adder roller and doctor blade.
U.S. Pat. No. 4,445,771 (Sakamoto et al.), and its divisions U.S. Pat. No. 4,575,218 and U.S. Pat. No. 4,576,463, disclose applying magnetic toner by a magnetic adder sleeve to a particle electrode developer sleeve, and forming it into a layer by a doctor blade, under magnetic brush charging. Contact or non-contact developing is effected under a developer sleeve voltage bias to prevent toner deposition on the image background area, or under friction charging of the developer sleeve to the same polarity as the background area for the same purpose. For nonmagnetic toner, the adder sleeve is omitted.
U.S. Pat. No. 4,710,015 (Takeda et al.) discloses applying nonmagnetic or magnetic toner by an adder roller to a particle electrode developer roller for contact developing under a developer roller voltage bias. The particle electrode layer on the developer roller is overcoated with an insulating outer coating.
U.S. Pat. No. 4,459,009 (Hays et al.) discloses gravity flow of nonmagnetic insulating toner between an adder roller and developer roller, both of opposite polarity to the toner, the adder roller having a triboelectrically active coating of such opposite polarity. A layer of toner forms on the developer roller that spaces the two rollers. Contact developing occurs under an adder roller bias of the same polarity as the toner, e.g., +100 VDC, and an opposite polarity developer roller bias, e.g., -250 VDC, at an opposite polarity latent image charge, e.g., -500 VDC, and opposite polarity background area discharge level, e.g., -100 VDC.
U.S. Pat. No. 4,764,841 (Brewington et al.), which is related to said U.S. Pat. No. 4,459,009, concerns alternative gravity flow of the nonmagnetic toner directly onto a developer roller having a triboelectrically active coating of such opposite polarity. Contact developing occurs under a developer roller DC bias, and where an adder roller is optionally also used as in said U.S. Pat. No. 4,459,009, under another DC bias applied thereto.
U.S. Pat. No. 4,743,937 (Martin et al.) discloses applying nonmagnetic toner by an adder brush to a developer roller, both having triboelectrically active surface material of opposite polarity to the toner. A layer of toner is formed on the developer roller by a doctor blade optionally also of such triboelectrically active material. Contact developing of a latent image of such opposite polarity is effected under a developer roller DC bias and optionally a brush DC bias.
U.S. Pat. No. 4,774,541 (Martin et al.) discloses a unit like that of said U.S. Pat. No. 4,743,937, but with a cage roller of triboelectrically active material instead of a brush.
It is desirable to provide a gray scale monocomponent nonmagnetic development system for an electrophotographic device, e.g., a laser printer, effective for gray level or continuous tone contact or non-contact developing of the latent image in black and/or color printing, with a developer roller having a semiconductive layer, either coated with a non-conductive outer coating or uncoated.